Arguably the best place to learn the intricacies behind the controversial subject of terroir isn't a vineyard in Napa. It's in a homey warehouse space on San Francisco's Folsom Street.

Terroir, a shop and bar that bills itself as a natural wine merchant, slipped quietly through the back door of the city's wine scene at the end of last year. After a steady succession of new wine bars in nearly every neighborhood, it seemed like the concept had reached saturation.

For Guilhaume Gerard, Dagan Ministero and Luc Ertoran, the young, passionate proprietors of Terroir, however, the crowded San Francisco market still had an opening. The trio felt that "natural" wine - a sort of catchall term for organic, biodynamic and minimal intervention - was hardly represented.

"There was no place in this city that grouped all these wines together," says Gerard. "Just some here, some there."

Gerard and Ministero met a few years ago at Chez Spencer, where the two worked as servers. They both liked wine, and bonded over producers they admired, but neither of them knew much about natural winemaking. Soon after, Gerard returned to his native Paris, where he immersed himself in the natural wine bar scene. It was at La Muse Vin, a well-known bar on the Paris natural wine scene, where he had the bottle of Foillard Morgon that "changed my life."

When he returned to San Francisco, Gerard met up again with Ministero; the two brought in their friend Ertoran and began to lay the plans for Terroir, which they opened in late 2007. The timing has been perfect - whether or not Terroir is riding a green wave, the shop has grown exponentially in the past year, drawing patrons from the Bay Area and beyond.

"Our list is about 700 selections now, and it's still growing," says Gerard. "Since we started with around 70 wines, that's not too bad. We need to keep our customers interested, but we need to keep ourselves interested too. An important cellar, that's what we're aiming for, that's our goal."

Q:Natural wine seems like an open-ended term. How would you define it?

Ertoran: Dry farming, no herbicides or pesticides, no spraying. And using natural yeasts - that's the most important thing, I think. And being smart about sulfur.

Gerard: Look at it this way, the Romans were using sulfur, but not clones like Pommard or 777.

Q:How does that differ from organics or biodynamics?

Gerard: It's a philosophy where you don't look for labels, you look for people doing the right thing, making wine from their heart.

Q:The three of you are equal partners. How do you handle buying decisions?

Ministero: We all do the buying - it's like having a three-person tasting panel.

Q:You don't carry much domestic wine. Why?

Gerard: There's just not such history in the States yet. I mean, yeah, we feel overwhelmed by the commercialization of California wine, but it takes time, it's a cultural thing to have wine be such a part of life, to spend centuries working a piece of ground. There hasn't been the evolution.

Ertoran: On top of expressing terroir, our palate is not geared toward domestic wines.

Q:Why did you open a place like Terroir in San Francisco?

Gerard: What better place is there? It's a good market. And the challenge of turning Zinfandel drinkers into Beaujolais drinkers is beautiful. Also, I went to too many places in this city where they'd carry these kinds of wines and the staff would say like, "Oh, that's tasty." That's it! It's like they didn't do their homework.

Q:So how has the reaction from the local wine industry been?

Ertoran: That's mixed. I mean distribution, they're happy, we're moving wine that might be hard to sell someplace else. So people we are dealing with, and our customers, they are very happy.

Gerard: Our clientele goes from Alice Waters to Daniel Patterson - the chefs, servers, they like us. A lot of our customers get to know each other. The serious wine lovers, the geeks, they all hang around. It's a little community.

Q:Which wines, if any, served as an epiphany for you?

Ertoran: The Domaine de Peyra 2002 La Roche Cotes d'Auvergne. It was my first natural wine and I was like, wow.

Ministero: It's tied between Joly's Clos de la Coulee de Serrant and Patrick Jasmin's Cote Rotie. The light just went on with both of those wines.

Q:Which wines do you drink when you go out?

Gerard: We like to go where there's a good list. Sure, we bring wine but we always order a bottle off the list. Not all natural wines are good and not all conventional wines are bad. Like (Barolo producer) Bartolo Mascarello. It's not on the list here but I have six bottles at home because it's made in a very traditional style.

Q:What should someone expect if they were to walk in here off the street?

Ministero: The wine here has to have an identity. It's about identity and truth. I don't have to sell you any s- because I believe in the wines we carry.

Gerard: We're a cave a vins - the goal is to create the best collection of Italian and French natural wines in the world. You can buy wine, you can eat and drink here.